1. Field of the Invention
The present invention relates to a honeycomb structure.
2. Description of the Related Art
Conventionally, a common honeycomb catalyst used for conversion of automotive exhaust gas is manufactured by having a high specific surface area material such as activated alumina and catalyst metal such as platinum carried on the surface of a monolithic cordierite-based honeycomb structure having a low thermal expansion characteristic. Further, alkaline earth metal such as Ba is carried as a NOx occlusion agent for processing NOx in an oxygen-excess atmosphere such as a lean-burn engine or a diesel engine. In order to achieve a further improvement in conversion performance, it is necessary to increase the probability of contact of exhaust gas and the catalyst noble metal and the NOx occlusion agent. This requires the carrier to have a higher specific surface area and the particles of the noble metal to be reduced in size and highly dispersed. However, a mere increase in the carried amount of a high specific surface area material such as activated alumina only results in an increase in the thickness of the alumina layer, thus causing a problem in that the probability of contact is not increased or that pressure loss is too high. Accordingly, some modifications have been made on cell shape, cell density, and wall thickness (for example, see JP-A 10-263416). On the other hand, as a honeycomb structure formed of a high specific surface area material, a honeycomb structure formed by extrusion molding using a composition including inorganic fibers and an inorganic binder is known (for example, see JP-A 5-213681). Further, a honeycomb structure formed by joining honeycomb units through an adhesion layer in order to increase the size of such a honeycomb structure is known (for example, see DE 4341159 A1).
The contents of JP-A 10-263416, JP-A 5-213681, and DE 4341159 A1 are incorporated herein by reference in their entirety.